Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The microlithography process is carried out in what is referred to as a projection exposure apparatus having an illumination system and a projection objective. In that case the image of a mask (=reticle) illuminated via the illumination system is projected via the projection objective on to a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective in order to transfer the mask structure on to the light-sensitive coating on the substrate.
Mirrors are used as optical components for the imaging process in projection objectives designed for the EUV range (that is to say at wavelengths of for example about 13 nm or about 7 nm), due to the general lack of availability of suitable translucent refractive materials. In operation of such projection objectives which are designed for EUV, in which mask and wafer are usually moved relative to each other in a scanning process, because the positions of the mirrors which are in part movable in all six degrees of freedom both relative to each other and also relative to the mask or wafer, it may be desirable for their positions to be adjusted and maintained with a high level of accuracy to avoid or at least reduce aberrations and adverse effects that they entail in the imaging result.
It is known, in addition to a carrier structure which carries interchangeable elements such as aperture plates or actuators, to provide a measurement structure which is typically arranged outside the carrier structure and which is intended to ensure thermally and mechanically stable fixing of position sensors or other measurement systems for determining the mirror positions. A projection exposure apparatus having a measurement frame and a load-carrying frame is disclosed, for example, in EP 1 278 089 B1.
WO 2006/128713 A2 and US 2008/0212083 A1 disclose, among other things, a measurement arrangement in a projection objective of a microlithographic projection exposure apparatus, where the measurement arrangement measures the spatial relationship between at least two components of the imaging system, and a reference element is mechanically mounted directly to one of the components.
EP 1 465 015 A1 discloses, among other things, a projection exposure apparatus designed for EUV where the relative positions of the mirrors with respect to each other are measured directly by an interferometric mechanism or a capacitive mechanism.
U.S. Pat. No. 6,727,980 B2 discloses implementing system adjustment with higher wavelengths different from the actual working wavelength.